Method for detecting an aberration-compensated image

ABSTRACT

This invention relates to a method for sharply detecting an image which comprises, preparing a hologram with an aberrationfree wave front and an aberration wave front, illuminating the hologram with a wave front produced as a result of removing only the change in light path length caused by a phenomenon to be measured or of adding newly the change in the light path length from or to the aberration front, and effecting filtering such as the schlieren method or phase difference method at the focus of the aberration-free wave front against the change in the wave front caused by the phenomenon and superimposed on the aberration-free wave front.

United States Patent [72] Inventors Todao Tsurutn;

Yoshinobu Ito, both of Tokyo, Japan [21] Appl. No. 831,561 122] FiledJune 9, 1969 [45] Patented Aug. 24, I97] [731 Assign-tee Nlppon KognkuK.l(.

Tokyo. Japan [32] Priority June 21, 1968 [33] Japan [31 42530/1968 [54]METHOD FOR DETECTING AN ABERRATION- COMPENSA'I'ED IMAGE 3 Claims, 3Drawing Figs.

[52] US. Cl 350/36, 350/l62 SF, 356/129 [51 Int. Cl G02b 27/38 G02b27/00 [50] Field of Search 350/35, I62 SF; 356/l06, 107, 129; 73/675 H[56] References Cited UNITED STATES PATENTS 2,655,074 10/1953 Eckert etal. 356/107 3,501,216 3/1970 Kogelnik Assistant Examiner-Ronald J. SternAn0meyAnton J. Wille ABSTRACT: This invention relates to a method forsharply detecting an image which comprises, preparing a hologram with anaberration-free wave front and an aberration wave front, illuminatingthe hologram with a wave front produced as a result of removing only thechange in light path length caused by a phenomenon to be measured or ofadding newly the change in the light path length from or to theaberration front, and effecting filtering such as the schlieren methodor phase difference method at the focus of the aberration-free wavefront against the change in the wave front caused by the phenomenon andsuperimposed on the aberration-free wave front.

PATENTED M1824 IBYI FIG.2

METHOD FOR DETECTING AN ABERRATION- COMPENSATED IMAGE This inventionrelates to a method for detecting an image by means of filtering such asby the schlieren method or phase difference method after compensatingfor aberration.

In conventional schlieren method or phase difference method, when someaberrations remain in the optical system, especially when the objectbody or object phenomenon is placed within a container of complicatedshape, the point image or slit image to undergo filtering is expanded onthe focal plane due to aberrations caused by the container. Therefore,it is usual that the accuracy of detection of change in light pathdifference by means of schlieren method or phase difference method isextremely poor compared with that of an aberration-free optical system.

An object of this invention is to offer a method which removes theaberrations remaining in the optical system and, enables high accuracydetection of the change in light path difference by selectively pickingup only the difference in light path length produced by inserting aphenomenon or an object in the light path and superimposing the pickedup difference on the aberration-free wave front.

In conventional schlieren apparatus, it is not an easy task to removethe spherical aberration and astigmatism peculiar to the apparatus. But.according to this invention, it is possible to remove these disturbingaberrations as is clearly shown in the description of the exampledescribed later. Moreover, it is possible to measure with highsensitivity a phenomenon that takes place in a container of complicatedshape since the aberrations caused by the container can be removed.

This invention will be described more clearly referring to theillustrative embodiment shown in the attached drawings, in which:

FIG. I is a drawing showing the principle of this invention;

FIG. 2 is an optical layout of an example of this invention; and

FIG. 3 is a drawing showing the image at the focal plane of the exampleof FIG. 2.

First the principle on which this invention is based will be described.In FIG. 1, two light wave fronts l and 2, coherent with each other, areincident upon a dry plate 3 at a certain angle. After development, saiddry plate (called a hologram) is placed at the original position. Whenthe dry plate is illuminated by the wave front 1 the wave front 2 isregenerated at the original position and when the dry plate isilluminated by the wave front 2, the wave front 1 is regenerated at itsoriginal position. Assume that the wave front 1 is an aberration-freewave front and the wave front 2 a wave front containing aberrations, inaccordance with this explanation, it will be understood that a wavefront containing aberration has been converted into an aberration-freewave front. When a hologram prepared in this way is illuminated by alight formed by adding a slight change A of light path length to thewave front 2, a wave front formed by adding A to the aberration-freewave front I is reproduced.

In this way the aberrations of the optical system itself is completelyeliminated and only the change in the light path length that has beenintroduced between the hologram preparation (exposure) and theregeneration can be observed by schlieren method or phase differencemethod as the cause disturbing the aberration-free wave front Theabove-mentioned handling can be applied quite similarly to the case inwhich a hologram is prepared under the condition wherein a phenomenon isthen contained, and the said phenomenon is removed from the light pathat time of regeneration.

Next, an example of this invention will be described in detail referringto FIG. 2. In FIG. 2, the parallel light 1] from a coherent light sourceis divided into two directions by a half mirror 12, meet again through ahalf mirror 13 by way of reflecting mirrors M and M, respectively, andstrike a photographical dry plate 14, making an angle of 0 to eachother. On

the other hand, a container 16 such as a glass pipe is placed in lightpath 15. Then, a sharp point image 20 (refer to FIG. 3)

due to a light path 19 and an image 2! (refer to FIG. 3) that has passedthrough the light path 15 and has been expanded by the aberration causedby the container 16 can be obtained on the focal plane 18 of anobjective lens 17. If, in this arrangement, the dry plate 14 is exposedand placed to the original position after development and thenilluminated by only the light passing through the light path 15 byshielding the light path 19 with a shutter 24, the sharp point image 20is reproduced and, when a phenomenon 22 is placed in the con tainer 16,the disturbance of the wave front due to said phenomenon works todegrade the sharpness of the point image 20. When a knife edge 23 ismoved along a focal plane 18 to cut the focus 20 of the aberration-freewave front, a dark and light pattern is produced corresponding to thechange in light path length at the position of the phenomenon 22. Inother words, a schlieren image is obtained. If a phase plate is placedat the focus 20, a dark and light image showing the phase distributioncan be obtained in similar manner.

Since a point image having a favorable coherence is cut, a uniform fieldcan be obtained easily, and the sensitivity of the schlieren method orphase difference method becomes very high in order to pick up thephenomenon change which is superimposed on the aberration-free system.

We claim:

1. A method for eliminating the effects of aberrations introduced intoan optical system for detecting an object using a single coherent lightsource, the aberrations resulting from a cell containing the object andfrom imperfections of the optical system itself, comprising the stepsof:

splitting light from said source into an object beam which is directedthrough the cell onto a film positioned within the system and into areference beam which is passed directly onto the film along a pathangularly displaced with respect to the object beam, the cell having anempty state and another state containing the object, said reference beamcomprising an aberration free wave front;

exposing the film with the cell in one state;

developing the film;

restoring the developed film into essentially the same position withinthe optical system;

placing the cell into the other state;

reconstructing the reference beam by passing light from said coherentsource through the object beam path only; focusing with a lens thereconstructed beam in the rear focal plane of said lens; and

spatially filtering the reconstructed beam in said plane at the focalpoint of the reconstructed reference beam.

2. A method as in claim 1 wherein said filtering step comprises placinga knife edge in the focal plane at the focus of the reconstructedreference beam.

3. A method as in claim I wherein said filtering step comprises placinga phase plate at the focus of the reconstructed reference beam.

1. A method for eliminating the effects of aberrations introduced intoan optical system for detecting an object using a single coherent lightsource, the aberrations resulting from a cell containing the object andfrom imperfections of the optical system itself, comprising the stepsof: splitting light from said source into an object beam which isdirected through the cell onto a film positioned within the system andinto a reference beam which is passed directly onto the film along apath angularly displaced with respect to the object beam, the cellhaving an empty state and another state containing the object, saidreference beam comprising an aberration free wave front; exposing thefilm with the cell in one state; developing the film; restoring thedeveloped film into essentially the same position within the opticalsystem; placing the cell into the other state; reconstructing thereference beam by passing light from said coherent source through theobject beam path only; focusing with a lens the reconstructed beam inthe rear focal plane of said lens; and spatially filtering thereconstructed beam in said plane at the focal point of the reconstructedreference beam.
 2. A method as in claim 1 wherein said filtering stepcomprises placing a knife edge in the focal plane at the focus of thereconstructed reference beam.
 3. A method as in claim 1 wherein saidfiltering step comprises placing a phase plate at the focus of thereconstructed reference beam.